Diamond as a semiconductor material has an extremely large band gap of 5.5 eV, and a high carrier mobility of 2000 cm2/V·s at room temperature for both electrons and positive holes. The dielectric constant is also small, being 5.7, and the breakdown field is high, being 5×106 V/cm. Furthermore, diamond also has the rare characteristic of negative electron affinity in which the vacuum level exists below the lower end of the conduction band. Because of such excellent characteristics, diamond has potential applications as a material for semiconductor devices such as environment-resistant devices that operate in high-temperature environments or space environments, power devices capable of high-frequency and high-output operation, luminescent devices capable of emitting ultraviolet light, or electron emission devices capable of low-voltage operation.
A p-type semiconductor can be fabricated by doping diamond with boron. The method disclosed in Japanese Laid-Open Patent Application No. 10-081587 for doping with phosphorus makes it possible to reliably fabricate an n-type semiconductor. However, the acceptor level of 0.37 eV formed by boron doping, and the donor level of 0.57 eV formed by phosphorus doping are low compared to a silicon semiconductor or the like, and the resistance at room temperature (300 K) is high. Typically, the specific resistance at 300 K is about 105 Ωcm when the activation energy near room temperature is approximately 0.6 eV as calculated from the temperature dependence of the carrier concentration in phosphorus-doped diamond. Therefore, a method such as the one disclosed in J. Vac. Technol. B 24(2) p. 967 (2006), for example, is attempted, whereby the specific resistance is lowered to 5 to 8×102 Ωcm by doping with phosphorus to a high concentration of 5 to 7×1019 cm−3, and using the product as an electron emitting material.